i have this error message "Output argument 'dvref' is not assigned on some execution paths."when executing this codefunction dvref =IC(V,I)

persistent Vref Vold Iold c; if isempty(Vold) Vold=0; %initialized only once in the beginningendif isempty(Iold) Iold=0; %initialized only once in the beginningendif isempty(c) c = .1; %initialized only once in the beginningendif isempty(Vref) Vref=.5; %initialized only once in the beginningend